ALSA: firewire-lib: flush completed packets when reading PCM position

By flushing all completed but not yet reported packets before reading
the PCM hardware position, the granularity of the pointer is improved
from the interrupt interval to the packet size.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>